1. Field of the Invention
The present invention relates to a exhaust control valve assembly of a two stroke internal combustion engine. More particularly, the invention relates to a technically efficient exhaust control assembly wherein both the main exhaust control valve and the at least one auxiliary exhaust control valve can be controlled by only one actuator.
2. Discussion of the Related Art
As is generally known two-stroke (often referred also as two-cycle) engines generally have exhaust and intake ports in the side walls of a cylindrical chamber. The cylindrical chamber and its side walls are usually referred to simply as a cylinder. While the intake ports feed air into the cylinder, the exhaust ports guide burned gas out of the cylinder into the exhaust of the engine. Since the intake and exhaust ports are at a fixed positions on the cylinder wall, the intake and exhaust ports are opened and closed by the piston which reciprocally moves up and down in the cylinder. Ideally however, for optimal operation of the engine, the position of the intake and the exhaust ports would be alterable according to the status of operation of the engine, preferably depending on engine speed and load. It follows from this consideration that the position of the exhaust ports is a compromise which allows for a proper performance of the engine both in low and high engine speed ranges.
It is known that by altering the vertical position of the upper edge of the exhaust port on the cylinder wall so that at high engine speeds the exhaust can be kept open longer, will considerably improve engine performance.
In this respect, it is known for example from AT 380 537 B that a main exhaust slide valve can be fitted in the exhaust passage proximate to the cylinder head. This main exhaust slide valve will be supported in a guide opening that is generally radial relative to the cylinder. At its free face end the slide valve forms a control edge that conforms to the cylinder bore, so that when the control valve is in its advanced position, the exhaust outlet cross-section is partially covered in its vertical dimension. This results in a corresponding reduction of the exhaust period because of the delayed opening and earlier closing of the exhaust passage. Thus, the charging of the cylinder in the middle and lower speed ranges is improved.
In some cases, in order to improve the exhaust conditions at the beginning and at the end of the exhaust gas discharge, auxiliary lateral exhaust outlets, connected through lateral channels to the main exhaust passage, are provided in the cylinder. These auxiliary exhaust outlets may be closed in order to reduce the exhaust period. Typically this is accomplished by cylindrical shutoff valves. These shutoff valves can through the lateral channels and can rotate or slide between a closed position and an open position. They are mechanically connected to the main exhaust valve such that they are actuated thereby.
One disadvantage of this known exhaust period control system is its technically complex design. Locating rotatable cylindrical shutoff valves in the lateral channels (which must be supported so as to be rotatable), and actuating them via the main exhaust valve (whose pivot axis is offset by 90° relative to the axes of rotation of the shutoff valves) is considerable. Another is the simple fact that the auxiliary (cylindrical shutoff) valves must be driven by the main exhaust valve. It would be desirable to have them be separately driven to allow them to be opened at different speeds.
EP 0 141 650 A2 describes a lateral auxiliary exhaust outlet which is controlled by means of a shutoff slide valve. The valve is actuated as a function of engine speed, with it only opening at higher speeds. However, this design incorporates no main exhaust valve at all.
JP 7279675 A describes an engine having both a main exhaust port with a main exhaust valve and an auxiliary exhaust port with an auxiliary exhaust valve. Both valves are actuated by a single motor; the main exhaust via a lever system, the auxiliary exhaust valves via a complicated gear system. This system is expensive and takes up a great deal of space.
Thus there is a need in the art for an improved exhaust system for a two-stroke internal combustion engine.